Well, look at it this way. The oil residue is just that - Oil. The oil does not burn, that's why it's spit out. If you lean the engine, it'll turn higher RPM, but it'll still spit out the same amount of oil.

In other words, by leaning out the engine, you may be putting less oil through the engine per stroke, but you're getting a lot more strokes, so it will be about the same.

As for your prop theory, that is what you call a rule of thumb. It may work, but it's far from accurate. I run a graupner 12x7 on my 55AX and it's far from a dog with 15% Cool Power.